Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system, a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions. For example, a turbocharged engine system may include a low-pressure (LP) EGR system which recirculates exhaust gas from the exhaust system to the intake passage upstream of a turbocharger compressor. Accordingly, exhaust gas may be recirculated into a low-pressure air induction system (LP AIS) upstream of the compressor, resulting in a compressed mixture of fresh intake air and EGR downstream of the compressor. An EGR valve may be controlled to achieve a desired intake air dilution, the desired intake air dilution based on engine operating conditions.
However, turbocharged engine systems may also include a compressor bypass valve (CBV). Among other functions, the CBV may serve to reduce compressor surge during certain conditions by recirculating the intake mixture downstream of the compressor back to the intake passage upstream of the compressor. As a result, the intake mixture entering the compressor during open CBV conditions may include a higher proportion of EGR (e.g., a higher intake air dilution) relative to the intake mixture entering the compressor when the CBV is closed, because it includes the EGR/fresh air mixture recirculated from downstream of the compressor due to the open CBV as well as additional EGR from the LP EGR system. Therefore, if no action is taken to address this issue, the desired intake air dilution may not be achieved, and engine performance may be degraded.
The inventors herein have recognized the above issue and have devised various approaches to at least partially address it. In one example approach, LP EGR may be reduced upon opening of a CBV (e.g., opening of a CBV during tip out to reduce compressor surge). In this way, over-dilution of the intake charge may be reduced by recirculating less exhaust, or even no exhaust, to the intake passage during conditions where an air/EGR mixture is already flowing from downstream of the compressor to upstream of the compressor by way of the open CBV. Then, after closing of the CBV (e.g., closing of the CBV after a compressor surge risk estimate has fallen below a threshold), it may be determined whether intake air dilution has decreased below a threshold based on measurements taken by an intake oxygen sensor arranged downstream of the compressor. If so, depending on the threshold, little or no EGR may be present in the intake air, and thus EGR may be increased (e.g., to achieve a desired intake air dilution) without the risk of over-dilution of the intake charge (which may undesirably degrade engine performance). As a further advantage of this approach, undesirable EGR backflow may be reduced during compressor surge conditions due to the reduction in opening of the EGR valve.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.